Implement modules as singletons Python semantics.

In Python, importing module several times returns same underlying module
object. This also fixes import statement handling for builtin modules.

There're still issues:
1. CPython exposes set of loaded modules as sys.modules, we may want to
do that either.
2. Builtin modules are implicitly imported, which is not really correct.
We should separate registering a (builtin) module and importing a module.
CPython keeps builtin module names in sys.builtin_module_names .

py/builtinimport.c

@@ -28,8 +28,13 @@ mp_obj_t mp_builtin___import__(int n_args, mp_obj_t *args) {
     }
     */
 
-    // find the file to import
     qstr mod_name = mp_obj_get_qstr(args[0]);
+    mp_obj_t loaded = mp_obj_module_get(mod_name);
+    if (loaded != MP_OBJ_NULL) {
+        return loaded;
+    }
+
+    // find the file to import
     mp_lexer_t *lex = mp_import_open_file(mod_name);
     if (lex == NULL) {
         // TODO handle lexer error correctly

py/obj.h

@@ -356,6 +356,7 @@ extern const mp_obj_type_t gen_instance_type;
 // module
 extern const mp_obj_type_t module_type;
 mp_obj_t mp_obj_new_module(qstr module_name);
+mp_obj_t mp_obj_module_get(qstr module_name);
 struct _mp_map_t *mp_obj_module_get_globals(mp_obj_t self_in);
 
 // staticmethod and classmethod types; defined here so we can make const versions

py/objmodule.c

@@ -17,6 +17,9 @@ typedef struct _mp_obj_module_t {
     mp_map_t *globals;
 } mp_obj_module_t;
 
+// TODO: expose as sys.modules
+static mp_map_t *loaded_modules;
+
 static void module_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
     mp_obj_module_t *self = self_in;
     print(env, "<module '%s' from '-unknown-file-'>", qstr_str(self->name));
@@ -46,14 +49,33 @@ const mp_obj_type_t module_type = {
 };
 
 mp_obj_t mp_obj_new_module(qstr module_name) {
+    if (loaded_modules == NULL) {
+        loaded_modules = mp_map_new(1);
+    }
+    mp_map_elem_t *el =  mp_map_lookup(loaded_modules, MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND);
+    // We could error out if module already exists, but let C extensions
+    // add new members to existing modules.
+    if (el->value != MP_OBJ_NULL) {
+        return el->value;
+    }
+
     mp_obj_module_t *o = m_new_obj(mp_obj_module_t);
     o->base.type = &module_type;
     o->name = module_name;
     o->globals = mp_map_new(1);
+    el->value = o;
     mp_map_lookup(o->globals, MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = mp_obj_new_str(module_name);
     return o;
 }
 
+mp_obj_t mp_obj_module_get(qstr module_name) {
+    mp_map_elem_t *el = mp_map_lookup(loaded_modules, MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP);
+    if (el == NULL) {
+        return NULL;
+    }
+    return el->value;
+}
+
 mp_map_t *mp_obj_module_get_globals(mp_obj_t self_in) {
     assert(MP_OBJ_IS_TYPE(self_in, &module_type));
     mp_obj_module_t *self = self_in;